Lessons From a Toad: How to Lighten Your Load

Jan. 11, 2006

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"A typical military pack weighs in excess of 80 pounds," Rome says. And today's Marine, or soldier, has to carry all sorts of electronic devices, like GPS and night vision goggles. But those all need batteries, and if your batteries run down in the hills of Afghanistan, there's no place to charge them. The result: about 20 extra pounds of batteries, including the necessary backups.

If the movement of the soldier could be tapped to produce electricity, that 20 pounds could be replaced by a rechargeable battery of about 1 pound. So the Navy asked Rome if he could figure out how to do that.

"I almost laughed," he recalls. "I have never really invented anything. It's like why is this guy asking me?"

But with Sept. 11 weighing heavily on the national consciousness, and on his as well, he wanted to do something. So he said he would give it a shot.

And he pulled it off.

Rome teamed with a couple of engineers at the University of Pennsylvania, and they created a backpack that uses rack-and-pinion technology to power a tiny generator. With a normal backpack, the load moves up and down about 2 inches with each step.

It's the upward movement that is the killer. A 50-pound backpack that is accelerated upward just 2 inches feels like an 85 pound backpack, if the wearer is simply walking. If the wearer is running, it feels like a 150-pound load.

"That's extraordinarily painful," Rome says. "It's bad news for people who hurt their knees and shoulders and other stuff."

Rome's backpack allows the load to slide up and down on the rack of the backpack, in effect remaining stationary relative to the distance from the ground as the wearer walks or runs. That energy produced by the movement of the load, relative to the backpack, is what drives the generator.

The result is a backpack that produces 7.4 watts of electricity, more than enough to run a host of electronic gizmos.

Rome and his colleagues have since focused their attention on reducing the workload for the wearer, and that has led to the ergonomic backpack. The load floats along the rack, remaining the same distance from the ground. That eliminates those costly peaks in energy, so a 50-pound backpack remains 50 pounds, even if the wearer is running, because it doesn't accelerate up with each step.

"In effect, the load stays in one place while the person runs around it," Rome says.

After he had already built a prototype, Rome ran across a description of Asian merchants who run down the street with a bamboo pole. Each end of the pole has a heavy load that remains about a foot off the ground because the pole flexes as the merchant runs. That eliminates the energy required to accelerate the load upward with each step.

"That's the same effect that we see," Rome says. Except he managed to stuff that long pole into a backpack.